| All-optical switches have attracted considerable interest recently because of their potential applications in optical communication, optical computing, optical information processing. The optical devices can break through the tramsmission speed limits of electro-optical, acousto-optical, thermo-optical and micro electromechanical switches. Such advantages spur people to devote the research of them. The all-optical switches based on optical Kerr effect use the third-order nonlinear refractive indices of materials, whose work principle can be described as follows. One laser beam inducing the change of refractive indices of materials, the signal beam transmitted in materials will produce accessional phase change to realizing the "On" or "Off" of optical switches. To make the practicable all-optical switches, nonlinear materials must simultaneously satisfy four main requirements: large nonlinear refractive index (n2) at the operating wavelength; minimal absorption losses due to onephoton or multiphoton absorption; ultrafast nonlinear refractive index responses and relaxation times (picoseconds or less); easy to make wave-guides. This dissertation takes the materials of which the all-optical switches are made as the object of the research.Current third-order NLO materials mainly focus on inorganic semiconductors, polymers and metal-organic complexes with delocalizedπ-electrons. Semiconductors possess NLO effects originating from saturable absorption, whose third-order NLO responses based on such resonant interactions may be relatively slow. Conjugated polymers and complexes possess large NLO responses, structural diversity and architectural flexibility (permitting molecular design and engineering), and facile use in thin films. But they also have obvious disadvantages: the materials are insulators with low electromigration rate, leading to slow NLO response of the integrated materials; have low energy transitions in the operating region and resonant nonlinearity. So few materials have been demonstrated that possess the switching speed, size of response, and low linear and nonlinear absorptions that are required. The limitations identified above spurred investigation of other novel materials.Transition metal DMIT (1,3-dithiole-2-thione-4,5-dithiolate) complexes have been widely studied for organic conductors and superconductors, and recently initiated the investigation on the nonlinear optical (NLO) properties. It has been found that these complexes possess largeπ-electron delocalization and charge transfer and high electromigration rate in the systems. So the characteristics should result in an ultrafast optical response capability and large third-order NLO effects. These make them into potential candidates in the NLO domain. Furthermore, the centra ions, the counter cations and oxidation state can be tuned according to some requirements. This provides feasibility to make all-optical switches.For our studying all-optical switches, besides ultrafast optical responses and large third-order NLO effects, materials need minimal absorption losses. To evaluate the suitability of a material for the kind of all-optical integrated devices, two figures of merit are defined: W = n2I/α0λand T =βλ/n2. According to the requirements thedevices need, it is necessary to achieve |W|>>1 and |T|<<1. But most of the DMITcomplexes are reported to possess resonant third-order NLO effects rooting in nonlinear absorption (β) of the complexes due to low energy transitions in the UV-Vis-NIR region. This lowers the figure of merit. So to explore new materials suitable for all-optical switches, this dissertation probes into the rules of effect of the central metal ions and the counter cations on the structures and third-order NLO properties, and intends to improve the third-order NLO refractive index and lower the absorption losses of the DMIT complexes through tuning the central metal ions and the counter cations. For this purpose, the dissertation mainly comprises the following aspects.Firstly, the dissertation studies the synthesis and preparation of the complexes and characterizes the structures of the materials. It is found that the central metal ions and the counter cations obviously influence the preparation and the structures of the materials. For the metal of the front half part in the transition system it is difficult to prepare the DMIT complexes due to the larger ratio of the charge to the radius. On the other hand, from VIII to IIB metal, the stability of the forming DMIT complex increases, form the planar and twisted planar structures to quasi-tetrahedral structures. The counter cations play an important role on stabilizing the complexes. With the counter cation being large, on the one hand, the complexes become easy to precipitate and separate, on the other hand, difficult to crystallize. The effect of the counter cations on the structures of the DMIT complexes is complicated and for different metals systems there are different things.Secondly, the dissertation investigates the third-order NLO properties of the DMIT complexes by Z-scan method. Through plentiful curves and data it is found that the central metal ions can obviously influence the third-order nonlinear refractive indices and nonlinear absorption of the materials in near IR area, strong nonlinear absorption for [Q]m[Co(dmit)n] and [Q]m[Ni(dmit)2] while relative larger third-order nonlinear refractive indices for [Q]2[Cu(dmit)2] and [Q][Au(dmit)2], especially the latter. The effect of the counter cations on the NLO properties of the complexes is complicated and there is different phenomenon for different metal, which is accordant with the effect of them on the structures of the complexes.Thirdly, the two complexes fit for the all-optical switches are found. Through an elementary investigation on [Q][Au(dmit)2], [Me4N][Au(dmit)2] possesses a large third-order nonlinear refractive index, n2=-4.11×1012 esu in solution (4×10-3 mol/L), and minimal nonlinear absorption with 40 ps at 1064 nm. The figures of its merits are calculated and |W| =2.87>>1, |T|≈0<<1. These data indicate that [Me4N][Au(dmit)2]is fit for all-optical switches. In addition, (BuPh3P)[Au(dmit)2] also has a large third-order nonlinear refractive index, n2=-16.06×10-12 esu in 2×10-3 mol/L solution, and relative less nonlinear absorption with 28 ps at 532 nm. The figures of its merits (|W| =0.73, |T| =0.64<1) are worthy of farther studing for all-optical switches.Fourthly, the dissertation simply discusses the relation between the third-order NLO properties and the structures. It is found that there are not NLO responses for the tetrahedral structures for IIB-matals and obvious nonlinear absorption for the square-planar configuration of VIII-matals, and there are large third-order nonlinear refractive indices for the twisted planar structures for IB-matals, especially [Q]Au(dmit)2]. These show that the twisted planar structure is a base for exploring the materials for all-optical switches.Fifthly, the mechanism responsible for the nonlinear absorption of [Q]m[Co(dmit)n] and [Q]m[Ni(dmit)2] is analyzed using a five energy-level diagram. It is found that the distinct excited state absorption behaviors were observed in different complexes: a saturable absorption (SA) for [Q]m[Ni(dmit)2] while a two photon absorption (TPA) for [Q]m[Co(dmit)n] with 40 ps laser pulses at 1064 nm. On the other hand, the nonlinear absorption responses of [Q]2[Cu(dmit)2] with different wavebands are investigated, and it is observed that there are a RSA in Vis area and a TPA in near IR area. The different NLO mechanism can help us exploit different types of the all-optical devices in addition all-optical switches.In addition, the composite film of DMIT complex-SiO2 is simply prepared by sol-gel process. It is proved that the process is feasible by SEM and the linear absorption spectrum.
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